1. Field of the Invention
The present invention relates to a production process for producing a hollow steel tube of a high strength which is applicable to automobile axles, or the like.
2. Description of the Related Art
There have been the lightweight requirements for vehicles such as automobiles, or the like. Accordingly, axles have been examined to hollow them out, and part of such attempts have been put into actual applications. The specifications of such hollow axles which have been put into actual applications by certain automobile manufactures are identical with each other more or less, and they can be summarized as follows:
Material Quality: S45C through S50C (as per Japanese Industrial Standard), carbon steel for mechanical structures, or the equivalents;
Hollowing Rate: approximately 50%, and the ratio of the inside diameter (hereinafter abbreviated to "I.D.") to the outside diameter (hereinafter abbreviated to "O.D."), i.e., I.D./O.D., falling in a range of from 0.45 to 0.55;
Hollowing Way: Drilling round bar stocks with a gun drill followed by reaming; and
Heat Treatment: Induction hardening followed by tempering (Here, the hardening is carried out up to about a half of the thickness of the hollow axles.)
The S45C through S50C carbon steels were used to make the conventional hollow axles because the inner hardness was intended to be as high as possible in the unhardened portions while maintaining such high carbon contents that there arise no cracks resulting from the hardening.
Namely, when the induction hardening is carried out too deep, the crystalline grains become so coarse that the resulting hollow axles are deteriorated in the impact strength. Accordingly, the heat treatment is limitedly carried out only up to about a half of the thickness of the conventional hollow axles. Therefore, the conventional hollow axles hardened to the shallow depths cannot be applied to large size vehicles which produce high outputs, because they lack the torsional fatigue fracture resistances. It has been known that the torsional fatigue fracture resistances of the automobile axles depend greatly on the induction hardened depth. FIG. 17 illustrates the relationship between the torsional resistances and the hardened depths, relationship which was exhibited by the conventional hollow axles hardened to such shallow depths. In FIG. 17, solid diamonds ( ) designate the torsional resistance exhibited by the conventional hollow axles made from "TS" No. 1, and blank circles (.largecircle.) designate the torsional resistance exhibited by the conventional hollow axles made from "TS" No. 3. As illustrated in FIG. 17, the conventional hollow axles exhibited sharply deteriorating torsional resistances when the ratios of the hardened depths to the radii were 0.6 or less. Here, the conventional hollow axles were examined for the torsional resistances by twisting them with an increasing torque until they ruptured or broke.
With respect to the hollowing way, the conventional hollow axles were bored by machining the round bar stocks because there are no alternative ways available. In view of the productivity, it takes a long processing tact-time for such boring with the drill to complete hollowing out a round bar stock, and consequently it is needed to provide a plurality of the facilities therefor. As a result, the manufacturing cost for the conventional hollow axles increases sharply because of the costs of the facilities and the spaces to be secured in a manufacturing plant. Thus, it is impossible to actually hollow out all the axles for all types of vehicles currently manufactured.
As for the other hollowing way, one can think of using electrically-welded steel tubes, for example. However, it is impossible to produce electrically-welded tubes which have heavy thicknesses appropriate for the manufacture of the vehicle axles, because they are formed into the tubular shapes by welding thin steel strips.
Further, the production process for seamless steel tubes can make hollow steel tubes of desired sizes, e.g., I.D.'s. and O.D.'s., at the least expensive cost. However, in the manufacture of the seamless steel tubes, it is inevitable that the seamless steel tubes contain flaws in the peripheries. The flaws in the outer peripheries of the seamless steel tubes can be machined out in one of the manufacturing processes of the vehicle axles, but the flaws in the inner peripheries thereof are left as they are in the end. In addition, no analysis has been made extensively so far in order to verify how wide and deep the flaws can be so as not to render them the starting points of fatigue fracture in the resulting hollow steel tubes.
For instance, the present inventors examined the seamless steel tubes, which had I.D.'s and O.D.'s applicable to hollow axles, for the flaws, and they found that the seamless steel tubes contained the flaws of 0.3 mm in depth. These flaws are believed to result from the scratches which are caused by a rolling and boring machine in the boring process of the seamless steel tubes production process. Namely, the scratches are believed to be folded into the inner peripheries of the seamless steel tubes when the seamless steel tubes are drawn and rolled to a predetermined thickness, i.e., to a half of the differences between O.D.'s and I.D.'s, by a stretching reducer, and they are believed to be left in the inner peripheries as the flaws of such a deep depth.